1. Field of the Invention
The present invention generally relates to a printer and particularly to improvements of a platen of the printer.
2. Related Art Statement
There is known a serial printer, such as a serial dot printer, which includes (a) a recording head which is displaceable along one of opposite surfaces of a recording medium, such as a multiple-sheet recording medium consisting of a set of superposed recording sheets, and (b) a platen which is opposed to the recording head via the recording medium positioned therebetween.
In a serial dot printer, such a multiple-sheet recording medium is used which includes a non-copying, top sheet and one or more copying sheets on which the top sheet is provided and on each of which a copy of an original being recorded on the top sheet is produced simultaneously with the recording of the original. The copying sheets are, for example, a pressure sensitive sheet, or a carbon-copying sheet on which a carbon copy is produced using a carbon paper or layer interposed between the top sheet and the copying sheet. However, the greater the number of the superposed sheets of a multiple-sheet recording medium is, the more difficult it is to produce a clear copy on every copying sheet down to the sheet or sheets at the bottom of the recording medium.
If the serial dot printer is modified to cause the dot recording wires of a recording head thereof to impact with increased force against the multiple-sheet recording medium, one or more holes might be opened through the top sheet of the recording medium, or the top sheet might even be torn. Conversely, if the wires of the recording head are adapted to impact with reduced force against the recording medium, the reduced impact force might not be transmitted down to the copying sheet or sheets at the bottom of the recording medium, so that an unclear copy may be produced on the bottom sheet or sheets.
In the first case where the increased impact force of the recording head is used for effecting recording on the recording medium, noise is increased, which causes people around the printer to feel discomfort. In addition, the printer needs to employ such a platen which is proof against the increased impact force, which leads to enlarging the size of the printer as a whole.
In the above described background, Japanese Patent Application laid open for opposition under Publication No. 56(1981)-41429 disclosed an impact printer which includes a recording head and a platen which is opposed to the recording head via a recording medium positioned therebetween. The Japanese document states that, when the recording head impacts against the platen for effecting recording on the recording medium, an ultrasonic vibrator or a high-frequency electric power source (in the latter case, is a piezoelectric element is provided in the platen) operated for vibrating the platen at a high frequency.
In the impact printer, the vibration force of the platen is applied to a recording medium in addition to the impact force of the recording head, so that a clear copy may be produced on every copying sheet included in a multiple-sheet recording medium. In addition, since the impact force of the recording head need not be increased, the printer does not increase noise and therefore does not cause people to feel discomfort.
However, in the prior printer, the platen is not displaceable, and the entire platen or respective portions of the same is/are vibrated at high frequencies. Thus, the printer consumes excessively large amount of energy for vibrating the platen, and therefore vibrating the platen costs very high. In addition, since the printer needs to employ the vibrator or the electric power, the size of the printer as a whole is adversely increased.
Furthermore, in the case where a piezoelectric element consisting of stacked piezoelectric ceramic layers (these layers are adhered to one another) is used as an ultrasonic vibrator for vibrating a platen of a printer, flexural or bending vibration may be applied to the element in a direction perpendicular to the direction of stacking of the ceramic layers (hereinafter, referred to as the "layer-stack direction"). The bending vibration is applied to the piezoelectric element for the following reasons: First, one of the opposite ends of the piezoelectric element as viewed in the layer-stack direction thereof is fixed, and the other end of the element is allowed to vibrate, i.e., expand and contract in the layer-stack direction thereof. Second, a platen surface to which the other end of the piezoelectric element is connected may slightly be slanted with respect to an operative surface of a recording head of the printer, when the element is assembled with the platen. Third, the operative surface of the recording head may be slanted with respect to the platen surface, when the recording head is displaced along a recording surface of a recording medium. Fourth, the operative surface of the recording head may impact against the platen surface in a direction slanted with respect to the platen surface, depending upon characters or other image patterns to be recorded on the recording medium. For the above reasons, the impact force of the recording head may be applied in a direction slanted with respect to the platen surface, and therefore some component of the impact force (i.e., bending vibration) may be applied to the piezoelectric element in a direction perpendicular to the layer-stack direction thereof. Thus, tensile stress may be produced in an outer arcuate (i.e., elongated) portion of a curved part of the stacked ceramic layers of the piezoelectric element, so that peeling may occur between the ceramic layers in the elongated portion of the element and therefore that the element may not vibrate normally, i.e., may vibrate in directions different from the direction perpendicular to the operative surface of the recording head. In an extreme case, the piezoelectric element may break so that it may fail to vibrate the platen surface.